Process of forming epitaxial substrate and semiconductor device provided on the same

ABSTRACT

A process of forming a nucleus fanning layer in a nitride semiconductor epitaxial substrate is disclosed. The process includes steps of growing: a lower layer of the nucleus forming layer on a substrate; an upper layer of the nucleus thrilling layer on the lower layer; and a nitride semiconductor layer each by the metal organic chemical vapor deposition (MOCVD) technique. The growth of the nitride semiconductor layer is done at a temperature lower than a growth temperature for the upper layer, and the growth of the upper layer is done by supplying ammonia (NH 3 ) at a flow rate greater than the flow rate of ammonia (NH 3 ) timing the growth of the lower layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is closely related to a patent application Ser. No.15/820,596 filed on Nov. 22, 2017 by an applicant same with the presentapplication, which is hereby incorporated by reference. The presentapplication claims the benefit of priority of Japanese PatentApplication No. 2016-246993, filed on Dec. 20, 2016, which isincorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a process of forming an epitaxialsubstrate and a semiconductor device on the epitaxial substrate.

2. Related Prior Arts

An epitaxial substrate generally includes semiconductor layersepitaxially grown on a substrate by, for instance, the metal organicchemical vapor deposition (MOCVD) technique. Forming electrodes of asource, a drain, and a gate on thus prepared epitaxial substrate, asemiconductor device such as a field effect transistor (FET) may beobtained. A Japanese Patent Applications laid open No. JP2010-225710Aand JP2013-004681A have disclosed a process of epitaxially growing analuminum nitride (AlN) layer, an aluminum gallium nitride (AlGaN) layer,and a gallium nitride (GaN) layer sequentially on a substrate. Also, thelatter prior patent document, JP2013-004681A, has reported thatdissociation of nitrogen (N) from nitride semiconductor materials causesa drain leak current of an FET.

A process for forming an epitaxial substrate that includes an AlN layeron a substrate, where the AlN layer is sometimes called as a nucleusforming layer, and another nitride semiconductor layer grown on thenucleus forming layer occasionally differentiates growth conditions,especially a growth temperatures between the nucleus forming layer andthe nitride semiconductor layer on the nucleus forming layer. Such aprocess changes the temperature condition after the growth of thenucleus forming layer as ceasing supplement of source gases. During thechange of the temperature, the nucleus forming layer in a surfacethereof is exposed within a condition of a high temperature, whichaccelerates the dissociation of nitrogen (N) from the surface of thenucleus forming layer and causes defects in an interface between thenucleus forming layer and the nitride semiconductor layer.

SUMMARY OF INVENTION

An aspect of the present invention relates to a process of forming anitride semiconductor device. The process includes steps of sequentiallygrowing a lower layer, an upper layer, and a nitride semiconductor layeron a substrate by the metal organic chemical vapor deposition (MOCVD)technique, where the lower layer and the upper layer, each made ofaluminum nitride (AlN), operate as a nucleus forming layer. The step ofgrowing the nitride semiconductor layer may be done under a temperaturethat is lower than a growth temperature of the upper layer. A feature ofthe process according to the present invention is that the step ofgrowing the upper layer supplies a source gas for nitrogen (N), whichmay be ammonia (NH₃), with a flow rate that is greater than a flow rateof a source gas for nitrogen (N) in the lower layer.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 shows a cross section of a semiconductor device according to thefirst embodiment of the present invention;

FIGS. 2A to 2C show cross sections of a semiconductor epitaxialsubstrate directed to the semiconductor device shown in FIG. 1, wherethe cross sections corresponds to respective processes of forming theepitaxial substrate;

FIG. 3A shows a sequence of forming the epitaxial substrate by the metalorganic chemical vapor deposition (MOCVD) technique, FIG. 3B shows flowrates of ammonia (NH₃) during the growth of a lower layer and an upperlayer of a nucleus forming layer in the first embodiment, and FIG. 3Calso shows flow rates of ammonia (NH₃) in a process modified from theprocess shown in FIG. 3B; and

FIG. 4A shows flow rates of tri-methyl-aluminum (TAM) during the growthof the lower layer and the upper layer of the nucleus forming layeraccording to the second embodiment, FIG. 4B also shows a flow rate ofTMA in the process modified from that shown in FIG. 4A, and FIG. 4Cshows a flow rate of ammonia (NH₃) in the process according to the thirdembodiment of the present invention.

DESCRIPTION OF EMBODIMENT

FIG. 1 shows a cross section of a semiconductor device 100 according tothe first embodiment of the present invention. The semiconductor device100 of the embodiment, which is a type of high electron-mobilitytransistor (HEMT), provides an epitaxial substrate 110, a sourceelectrode 13, a drain electrode 15, and a gate electrode 17. Theepitaxial substrate 110 includes a substrate 10, a nucleus forming layer11, a channel layer 12 that is made of gallium nitride (GaN), a barrierlayer 14, and a cap layer 16, where the channel layer 12, the barrierlayer 14, and the cap layer 16 consists a semiconductor stack 19 ofnitride semiconductor layers. The cap layer 16 forms the electrodes ofthe source 13, the drain 15, and the gate 17 thereon. A feature of thesemiconductor device 100 of the first embodiment is that the nucleusforming layer 11 is divided into two layers, exactly the lower layer 11a and the upper layer 11 b, and the upper layer 11 b has a nitrogencomposition that is higher than the nitrogen composition of the lowerlayer 11 a.

Specifically, the lower layer 11 a is provided on and in contact withthe substrate 10, while, the upper layer 11 b is provided on and incontact with the lower layer 11 a. The GaN layer 12 is provided on theupper layer 11 b, the barrier layer 14 is provided on the GaN layer 12,and the cap layer 16 is provided on the barrier layer 14. The electrodesof the source 13, the drain 15, and the gate 17 are provided on the caplayer 16.

The substrate 10 may be made of silicon carbide (SiC). The lower layer11 a is made of aluminum nitride (AlN) with a thickness of 10 nm, while,the upper layer 11 b is also made of AlN but with a thickness thinnerthan that of the lower, which is 3 nm in the present embodiment.Although the lower layer 11 a may have a stoichiometric composition, theupper layer 11 b has a non-stoichiometric composition, exactly an N-richcomposition.

The channel layer 12 may be made of un-doped gallium nitride (GaN) witha thickness of around 600 nm, where a lower portion of the GaN layer 12may be operable as a buffer layer 12 a, while, an upper portion thereofmay be operable as a channel layer 12 b. The barrier layer 14, which maybe made of aluminum gallium nitride (AlGaN) with an aluminum compositionof 0.22, has a thickness of 24 nm and forms or induces the channel inthe channel layer 12 b at the interface against the barrier layer 14.The cap layer 16, which may be made of n-type GaN, has a thickness of 5nm.

The source electrode 13 and the drain electrode 15 may be formed from astacked metal of tantalum (Ta), aluminum (Al), and tantalum (Ta), whichmay be denoted as Ta/Al/Ta, from a side of the cap layer 16; that is,the source electrode 13 and the drain electrode 15 may be formed byalloying the stacked metal above described. The gate electrode 17 mayalso provide a stacked metal of, from the side of the cap layer 16,nickel (Ni), palladium (Pd), gold (Au), and tantalum (Ta), which may bedenoted as Ni/Pd/Au/Ta, where the nickel (Ni) makes a Schottky contactagainst the cap layer 16.

Table 1 below summarizes growth conditions of the epitaxial substrate110 for the semiconductor device 100. In table 1, TMA, TMG, NH₃, SiH₄mean tri-methyl-aluminum, tri-methyl-gallium, ammonia, and silane,respectively. Also in table 1, one (1) Torr is equal to 133.3 pascal(Pa), one (1) sccm (standard cc per minutes) is equal to 1.667×10⁻⁸m³/s, and one (1) slm (standard liter per minutes) is equal to1.667×10⁻¹¹ m³/s, respectively.

TABLE 1 growth conditions thickness pressure temperature source layer(nm) (Torr) (° C.) flow rates lower layer 11a 10 100 1100 TMA: 130 sccmNH₃: 15 slm upper layer 11b 3 TMA: 130 sccm NH₃: 20 slm GaN layer 12 6001060 TMG: 54 sccm NH₃: 20 slm barrier layer 14 24 TMG: 30 sccm TMA: 90sccm NH₃: 20 slm SiH₄: 8 sccm cap layer 16 4 TMG: 43 sccm NH₃: 20 slmSiH₄: 40 sccmThe lower layer 11 a and the upper layer 11 b are grown at a growthtemperature of 1100° C. and a growth pressure of 100 Torr, but the lowerlayer 11 a is grown by supplying ammonia (NH₃) with a flow rate of 15slm; while, the upper layer 11 b is grown with a flow rate of NH₃ of 20slm that is increased by 5 slm from that in the lower layer 11 a.

FIGS. 2A to 2C show cross sections of the epitaxial substrate 110 atrespective steps of the process of growing the epitaxial substrate 110.FIG. 3A shows a sequence of growth temperatures for the epitaxialsubstrate 110, FIG. 3B shows flow rates of ammonia (NH₃) during thegrowth of the lower layer 11 a, the upper 11 b and the GaN layer 12. InFIGS. 3A and 3B, hatched areas show periods where the respective layersare grown, while, non-hatched areas correspond to periods where thegrowths are interrupted.

The process first sets the temperature of the substrate 10 to be 1100°C. Supplying ammonia (NH₃) with the flow rate of 15 slm and TMA 10 withthe flow rate of 130 sccm, the lower layer 11 a made of AlN is grown onthe substrate 10 during the period from t₁ to t₂ by a thickness of 10nm. Then, interrupting the supplement of TMA, the process increases theflow rate of ammonia (NH₃) from 15 to 20 slm but continuously flowingnitrogen (N), which operates as a carrier gas, as keeping thetemperature of the substrate 10 to be 1100° C. during the period from t₂to t₃. At the instant t₃ where the flow of ammonia (NH₃) becomes stable,the growth of the upper layer 11 b begins by supplying TMA with the flowrate of 130 sccm until the upper layer 11 b has a thickness of 3 nm.Thus, the process decreases a ratio of the flow rates of a source gasfor the group III element, namely aluminum (Al), against the flow rateof the source gas for the group V element, namely nitrogen (N), wherethe ratio of the flow rates of the source gases is often denoted as aIII/V ratio.

Thereafter, the process lowers the temperature of the substrate 10 downto 1060° C. during a period from t4 to t5, as shown in FIG. 3A. Duringthat period, the supplement of ammonia (NH₃) is interrupted but nitrogen(N) as the carrier gas is continuously supplied as shown in FIG. 3B.Setting the flow rate of ammonia (NH₃) to be 20 slm, and supplying thegas sources for the group III element as shown in Table 1 above, the GaNlayer 12 may be grown on the upper layer 11 b. The process further growsthe AlGaN barrier layer 14 on the GaN layer during the period t7 to t8in FIG. 3A, and the GaN cap layer 16 on the AlGaN barrier layer 14during the period t9 to t10. The process continuously supplies nitrogen(N) as a carrier gas during the growth of the semiconductor stack 19including the GaN layer 12, the AlGaN barrier layer 14, and the GaN caplayer 16. Thus, the epitaxial substrate 110 may be formed. Forming theelectrodes of the source 13, the drain 15, and the gate 17 on the caplayer 16, the semiconductor device 100 shown in FIG. 1 may be obtained.The semiconductor device 100 thus formed may further provide apassivation film made of, for instance, silicon nitride (SiN) to protectthe epitaxial substrate 110 and the electrodes, 13 to 17, on a top ofthe epitaxial substrate 110. In a modification, the semiconductor device100 may omit the cap layer 16 and provide the passivation film describedabove on the barrier layer 14, where the electrodes, 13 to 17, may bedirectly provided on the barrier layer 14.

The first embodiment of the process of forming the semiconductor device100 thus described increases the supplement, or the flow rate of ammonia(NH₃) as the source gas for nitrogen (N) during the growth of the upperlayer 11 b, which means that the nitrogen composition in the upper layer11 b becomes larger compared with that of the lower layer 11 a, which issometimes called as an N-rich composition. Because the growthtemperature for the GaN layer 12 is lower than that for the upper layer11 b, the process is necessary to lower the temperature of the substrate10 after the growth of the upper layer 11 b but before the growth of theGaN layer 12, during which the substrate 10, exactly, the surface of theupper layer 11 b is exposed to a high temperature atmosphere and theupper layer 11 b possibly dissociates nitrogen (N) from the surfacethereof. The N-rich composition of the upper layer 11 b may effectivelycompensate the dissociation of nitrogen (N) and suppress defects causedby the dissociation of nitrogen (N), which may also suppress defectsinduced in the layers grown on the upper layer 11 b. Thus, the drainleak current caused by the defects may be effectively reduced.

A process may increase the supplement of ammonia (NH₃) during not onlythe growth of the upper layer 11 b but the growth of the lower 11 a toincrease the nitrogen composition. However, when the nucleus forminglayer 11 has an N-rich composition in a whole thereof, the growth forthe semiconductor stack 19 on the AlN nucleus forming layer 11 variesthe growth mode thereof and degrades crystal quality thereof. Thus, theprocess may divide the growth of the nucleus forming layer 11 into thelower layer 11 a on the substrate 10 and the upper layer 11 b on thelower layer 11 a, where the lower layer 11 a has a substantiallystoichiometric composition between aluminum (Al) and nitrogen (N), whilethe upper layer 11 b has the N-rich composition by increasing thesupplement, or the flow rate of ammonia (NH₃).

The nucleus forming layer 11 is preferably grown under relatively highertemperature to decrease pits appearing on a grown surface thereof.Accordingly, the growth temperature for the nucleus forming layer 11 ispreferably higher than 1100° C. While, the GaN layer 12 is preferablygrown under a relatively lower temperature to suppress the dissociationof nitrogen (N) from the surface thereof. Accordingly, the growthtemperature for the GaN layer 12 is preferably lower than that for thenucleus forming layer 11, typically around 1060° C. However, thoseconditions for the nucleus forming layer 11 and the GaN layer 12 areoptional. For instance, the growth temperature for the GaN layer 12 ispreferably lower than that for the nucleus forming layer 11 by at least30° C. The difference in the growth temperatures may be 40° C. orgreater in the present embodiment. The difference in the growthtemperature inevitably accompanies with a lowering of a temperatureafter the growth of the upper layer 11 b. As the difference in thegrowth temperatures becomes larger, a period to lower the temperatureand stabilize thereat becomes longer, which means that a period for theupper layer 11 b to be exposed in a high temperature becomes longer andnitrogen (N) is possibly dissociated from the surface of the upper layer11 b. The embodiment of the present embodiment, because of the N-richcomposition of the upper layer 11 b, may effectively suppress thedefects caused in the surface of the upper layer 11 b.

The supplement, or the flow rate of ammonia (NH₃) during the growth ofthe upper layer 11 b is preferably greater than 20 slm, which may formthe N-rich composition in the upper layer 11 b and suppress the defectsdue to the dissociation of nitrogen (N) from forming in the upper layer11 b. The process may increase the supplement, or the flow rate ofammonia (NH₃) as a difference in the growth temperatures between theupper layer 11 b and the GaN layer 12 becomes larger in order to enhancethe nitrogen composition in the upper layer 11 b. Preferably, the flowrate of ammonia (NH₃) for the upper layer 11 b is greater than that forthe lower 11 a by at least 5 slm.

In order to enhance the quality of the semiconductor stack 19 grown onthe nucleus forming layer 11, the semiconductor stack 19 preferablydecreases lattice miss-matchings against the substrate 10. Accordingly,the present embodiment provides the lower layer 11 a whose latticeconstant is closest to that of the substrate 10 and has a substantialthickness, which may be thicker than, for instance, 5 nm or a thicknessgreater than 10 nm is further preferable. The semiconductor stack 19should be grown on a semiconductor surface with lesser pits that couldaccelerate the dissociation of nitrogen (N). Accordingly, the upperlayer 11 b preferably has a thickness greater than 3 nm. The lower layer11 a may have a greater thickness than that of the upper layer 11 b.

The process of the present embodiment, as shown in FIG. 3B, continuouslysupplies ammonia (NH₃) during the period from t2 to t3, namely after thecompletion of the growth of the lower layer 11 a but before thebeginning of the growth of the upper layer 11 b. The growth temperaturesfor the lower layer 11 a and the upper layer 11 b are preferablyinvariant, because the process once interrupts the supplement of TMAafter the growth of the lower layer 11 a, increases the supplement orthe flow rate of ammonia (NH₃), and resumes the supplement of TMA againafter the flow rate of ammonia (NH₃) stabilizes. Accordingly, thesurface of the lower layer 11 a is temporarily exposed in a hightemperature during the period from t2 to t3. A condition where thegrowth temperatures for the lower layer 11 a and the upper layer 11 bare invariant may shorten the period from t2 to t3, which mayresultantly suppress the dissociation of nitrogen (N) from the lowerlayer 11 a.

FIG. 3C shows another sequence for supplying ammonia (NH₃) that ismodified from the sequence shown in FIG. 3B. That is, the sequence shownin FIG. 3C continues the supplement of ammonia (NH₃) in addition to thesupplement of nitrogen (N₂) as a carrier gas during the period after thegrowth of the upper layer 11 b but before the growth of the GaN layer12. The first embodiment described above interrupts the supplement ofammonia (NH₃) in addition to the TMA after the growth of the upper layer11 b. Modified from the embodiment described above, the sequence shownin FIG. 3C continues the supplement of ammonia (NH₃) and only the TMA isinterrupted after the growth of the upper layer 11 b. This sequence mayfurther suppress the dissociation of nitrogen (N) from the surface ofthe upper layer 11 b because, in addition to the upper layer 11 b withan N-rich composition, the atmosphere contains ammonia (NH₃).

Second Embodiment

A growth sequence according to the second embodiment of the inventiondecreases the supplement of TMA, which is a source gas for aluminum(Al), during the growth of the upper layer 11 b. Table 2 summarizes thegrowth conditions for the epitaxial substrate 110A according to thesecond embodiment of the invention.

TABLE 2 growth conditions thickness pressure temperature source layer(nm) (Torr) (° C.) flow rates 1^(st) layer 11a 10 100 1100 TMA: 130 sccmNH₃: 15 slm 2^(nd) layer 11b 3 TMA: 80 sccm NH₃: 20 slm GaN layer 12 6001060 TMG: 54 sccm NH₃: 20 slm barrier layer 14 24 TMG: 30 sccm TMA: 90sccm NH₃: 20 slm SiH₄: 8 sccm cap layer 16 5 TMG: 43 sccm NH₃: 20 slmSiH₄: 40 sccmAs clearly listed in Table 2 above, the lower layer 11 a and the upperlayer 11 b are grown under a temperature and a pressure common to thosetwo layers, 11 a and 11 b. The lower layer 11 a is grown by supplyingammonia (NH₃) with a flow rate of 15 slm, while, the upper layer 11 b isgrown with a flow rate of 20 slm increased by 5 slm from that for thelower layer 11 a. A feature of the second embodiment is that the flowrate of TMA for the upper layer 11 b is decreased from the flow rate ofTMA for the lower layer 11 a by 50 sccm, where the former is 80 sccmwhile the latter is 130 sccm.

FIG. 4A shows flow rates of TMA according to the second embodiment ofthe present invention. In FIG. 4A, the horizontal axis shows the period,while, the vertical axis corresponds to the flow rate of TMA. During thegrowth of the lower layer 11 a from the period t1 to t2, the flow rateof TMA is set to be 130 sccm as shown in FIG. 4A and listed in table 2.The growth of the upper layer 11 b from the period t3 to t4 is done bysetting the flow rate of TMA to be 80 sccm that is decreased by 50 sccmfrom the flow rate for the lower layer 11 a. Besides, the flow rate ofammonia (NH₃) during the growth of the lower layer 11 a from the periodt1 to t2 is set to be 15 slm as shown in FIG. 3B, while, that for theupper layer 11 b during the period from t3 to t4 is increased by 5 slmtherefrom, that is, the flow rate becomes 20 slm also shown in FIG. 3Band listed in table 2.

The second embodiment of the present invention may grow the upper layer11 b with the N-rich composition, which may reduce the drain leakcurrent. Besides, the flow rate of TMA during the growth of the upperlayer 11 b is decreased from that during the growth of the lower layer11 a, which may reduce the growth rate of the upper layer 11 b comparedwith the growth rate of the lower layer 11 a. Accordingly, the processmay precisely control a thickness of the upper layer 11 a. For instance,an upper layer 11 b having a limited thickness of 3 nm may be grownuniformly and precisely.

Modification of the second embodiment is shown in FIG. 4B that shows theflow rate of TMA for the nucleus forming layer 11. When the lower layer11 a is thinner than the upper layer 11 b, as shown in FIG. 4B, thelower layer 11 b may be grown by supplying TMA with a flow rate smallerthan that for the upper layer 11 b, for instance, setting the flow rateto be 80 sccm, which reduces the growth rate of the lower layer 11 a.That is, the flow rate of TMA may be decreased during the growth of oneof the lower layer 11 a and the upper layer 11 b with a thicknesssmaller than that of the other of the lower layer 11 a and the upperlayer 11 b. Then, the thickness of the layer grown under a smaller flowrate of TMA may be precisely controlled in a thickness thereof.

Third Embodiment

The third embodiment of the present invention gradually increases theflow rate of ammonia. FIG. 4C shows the flow rate of ammonia (NH₃)during the growths of the lower layer 11 a and the upper layer 11 b. Thethird embodiment of the present invention, as shown in FIG. 4C,gradually increases the flow rate of ammonia (NH₃) from the beginning ofthe growth thereof to the completion thereof, that is the flow rate ofammonia (NH₃) at the beginning is set to be 15 slm, while that at thecompletion of the growth is set to be 20 slm. Moreover, the flow rate ofammonia (NH₃) gradually and monotonically increases from the beginningof the growth to the completion thereof. According to the sequence shownin FIG. 4C, the upper layer 11 b may have the N-rich composition. Thus,the sequence shown in FIG. 4C may reduce the defects induced during thegrowth of the GaN layer 12 and the drain leak current. Continuous andmonotonic increase of the flow rate of ammonia (NH₃) may form atransition layer in an interface between the lower layer 11 a and theupper layer 11 b, which may reduce the defects induced within the upperlayer 11 b.

In the embodiment thus described, source materials and flow ratesthereof are not restricted to those exemplarily described above. Forinstance, the flow rate of ammonia (NH₃) may be greater than 20 slm orsmaller than 15 slm. Further specifically, the lower layer 11 a may begrown by supplying ammonia (NH₃) with the flow rate thereof smaller than15 slm, while, the upper layer 11 b may be grown by supplying ammonia(NH₃) with a flow rate greater than 20 slm. A key feature of theembodiment is that the flow rate of ammonia (NH₃) for the upper layer 11b is greater than that for the lower layer 11 a. Thus, the upper layer11 b may be grown as the N-rich composition, which may suppress thedissociation of nitrogen from the period t4 to t5, after the completionof the growth of the upper layer 11 b but before the beginning of thegrowth of the GaN layer 12, during which the surface of the upper layer11 b is exposed to an ambient of a high temperature. The source materialfor nitrogen (N) is not restricted to ammonia (NH₃), and that foraluminum (Al) is also not restricted to TMA. For instance,tri-ethyl-aluminum (TEA) may be applicable to a source material foraluminum (Al).

The semiconductor device 100 thus formed may further provide aninsulating film on the electrodes of the source 13, the drain 15, andthe gate 17, and on the cap layer 16 between the electrodes, 13 to 17,to enhance moisture resistance of the semiconductor device 100, wheresuch an insulating film is sometimes called as a passivation film. Theinsulating film may be made of silicon nitride (SiN), siliconoxy-nitride (SiON), and so on.

Also, the semiconductor substrate 10 of the embodiment may provide otherdevices except for the FET or an active device, that is, the substratemay provide or integrate passive devices. The substrate 10 may provideother electrodes except for the source 13, the drain 15, and the gate17, namely, those for passive devices and so on.

The semiconductor stack 19 includes nitride semiconductors, such as,except for GaN and AlGaN, indium gallium nitride (InGaN), indium nitride(InN), indium aluminum nitride (InAlN), indium aluminum gallium nitride(InAlGaN), and so on. The substrate 10 may be, except for SiC, made ofsilicon (Si), sapphire (Al₂O₃), gallium nitride (GaN), and so on.

Although the present invention has been described with reference tospecific embodiments, ordinary person skilled in the art will recognizethat changes may be made in form and detail without departing from thespirit and scope of the invention.

What is claimed is:
 1. A process of forming an epitaxial substrateincluding a substrate, the process comprising steps of: growing a lowerlayer made of aluminum nitride (AlN) with aluminum (Al) as a group IIIelement and nitrogen (N) as a group V element on the substrate by ametal organic chemical vapor deposition (MOCVD) technique at a sourcegas flow rate for the group III element and a source gas flow rate forthe group V element in a first III/V ratio; growing an upper layer madeof AIN by the MOCVD technique at a source gas flow rate for the groupIII element and a source gas flow rate for the group V element in asecond III/V ratio; and growing a nitride semiconductor layer on theupper layer under a growth temperature lower than a growth temperaturefor the upper layer, wherein the lower layer and the upper layer providean AIN nucleus forming layer, the upper layer having a thickness that is5 nm at most and that is smaller than a thickness of the lower layer onthe substrate, wherein the second III/V ratio is smaller than the firstIII/V ratio, and wherein ammonia (NH₃) continuously is supplied aftercompletion of growing of the lower layer and before beginning growing ofthe upper layer.
 2. The process according to claim 1, wherein the growthtemperature for the nitride semiconductor layer is at least 30° C. lowerthan the growth temperature for the upper layer.
 3. The processaccording to claim 1, further including a step of, after the step ofgrowing the upper layer but before the step of growing the nitridesemiconductor layer, supplying a source material for nitrogen (N) withinan apparatus for the MOCVD technique.
 4. The process according to claim1, wherein, in the step of growing the upper layer, the flow rate of thesource material for Al is decreased in comparison with the flow rate ofthe source material for Al in the lower layer.
 5. The process accordingto claim 1, wherein the step of growing the lower layer grows the lowerlayer with a thickness at least 5 nm.
 6. The process according to claim1, wherein the step of growing the nitride semiconductor layer grows alayer made of gallium nitride (GaN) under the growth temperature lowerthan the growth temperature of the upper layer.
 7. The process accordingto claim 1, wherein, in the step of growing the upper layer, the flowrate of the source material for nitrogen (N) is at least 5 standardliters per minutes (slm) and higher than the flow rate of the sourcematerial for nitrogen (N) during growth of the lower layer.
 8. Theprocess according to claim 1, wherein the upper layer has a nitrogenrich composition.